1. Field of the Invention
The invention relates to a manufacturing method of a rotor for an electric motor and a rotor for an electric motor that includes a core formed of stacked steel plates interposed between a first end plate, which is disposed on a lower side of the core, and a second end plate, which is disposed on an upper side of the core.
2. Description of the Related Art
Japanese Patent Application Publication No. 2004-32958 (JP-A-2004-32958) describes a structure of a rotor for an electric motor as a related art in which a core formed of stacked steel plates is pressed by end plates from both sides of the core in the stacking direction of the steel plates. The applicant has used the invention shown in FIG. 10, which shows the configuration of the rotor of the related art in which the core formed of the stacked steel plates is pressed by the end plates from the both sides in the stacking direction. A shaft 53 includes an inner shaft 53a and an outer shaft 53b formed on a bottom 53c. A flange portion 53d is formed on an outer periphery of the bottom 53c. A first end plate 55 and a core 54 are stacked on the flange portion 53d in the axial direction of the shaft 53. The core 54 includes a plurality of steel plates that are identically formed and stacked. The first end plate 55 and the core 54 are fitted onto the outer shaft 53b by inserting the outer shaft 53b through a center hole 55a of the first end plate 55 and through a center hole 54a of the core 54. A second end plate 52 is placed on and pressed against the core 54 at a pressure A so as to make the first end plate 55 closely contact the core 54, and also make the stacked steel plates of the core 54 closely contact each other. While the pressure A is applied to the second end plate 52, a pressure B is applied to a deformation punch 51 to deform a deformed portion 53e of the shaft 53, whereby the deformed portion 53e is deformed to fit a no core-side edge portion 52b of an inner edge of the second end plate 52 (this process will be hereinafter referred to as “deformation process”). The deformed portion 53e is thus deformed to fit the no core-side portion 52b of the inner edge of the second end plate 52, whereby the first end plate 55, the core 54, and the second end plate 52 are caused to closely contact each other.
However, the invention described in FIG. 10 adopts a configuration in which an inner portion 52c of the second end plate 52 is raised, compared to the position of an outer edge 52d, so that the inner edge of the second end plate 52 is easily deformed in an elastic manner when the deformed portion 53e is deformed to fit the no core-side edge portion 52b of the inner edge of the second end plate 52. FIG. 12 emphatically shows the feature of the actual configuration of the second end plate 52. The second end plate 52 is substantially inverted saucer shaped. More specifically, if the second end plate 52 is a flat plate, when the entire surface of the core 54 is pressed by the second end plate 52, a force is concentrated on the inner portion 52c of the second end plate 52, and the outer edge 52d of the second end plate 52 is lifted, and as a result, a gap is created between the outer edge 52d of the second end plate 52 and the core 54. Therefore, the configuration of the second end plate 52 has to be substantially inverted saucer shaped as shown in FIG. 12.
Because the process, in which the deformed portion 53e is pressed at the pressure B, involves the plastic deformation, the pressure B is four to five times higher than the pressure A at which the second end plate 52 is pressed. Therefore, even if the second end plate 52 is substantially inverted saucer shaped, because the high pressure B is applied to the inner portion 52c of the second end plate 52 through the deformed portion 53e as shown in FIG. 11, there is still a possibility that the inner portion 52c serves as a fulcrum and the outer edge 52d of the second end plate 52 is lifted, which results in a gap created between the outer edge 52d of the second end plate 52 and the core 54.
If there is a gap between the outer edge 52d of the second end plate 52 and the core 54, it is not possible to firmly press outer edges of the stacked steel plates of the core 54. If this occurs, a gap is created between the adjacent steel plates, so that magnetic resistance of the core 54 is increased, and lines of magnetic force generated are reduced. This reduces torque of the electric motor rotor. Further, if there is a gap between the outer edge 52d of the second end plat 52 and the core 54, it is not possible to firmly press the steel plates of the core 54, and the stacked steel plates therefore vibrate, resulting in unnecessary load to the electric motor rotor. This also reduces the torque of the electric motor rotor. Further, if the second end plate 52 is pressed excessively strongly from the no core-side edge portion 52b, magnets in the core 54 may be broken, and therefore, it is not appropriate to strongly press the second end plate 52 from the no core-side edge portion 52b. 